The present invention relates to a method and apparatus for measuring a concentration of a dye in arterial blood flowing in an artery in a living organism and, more particularly, to a method and apparatus for making measurements of dye concentration in arterial blood flowing in an artery in a noninvasive manner.
A method and apparatus for measuring dye concentration in arterial blood in living tissue can be used, for example, as part of a method of measuring cardiac output, i.e. the volume of blood pumped by the heart per unit time.
The measurement of cardiac output is an important clinical parameter which is routinely measured in clinical practice. A common procedure for measuring the cardiac output is based on the "indicator dilution" principle, namely: that the degree of dilution of the indicator injected in the circulating blood is a measure of the flow rate.
The method of measuring cardiac output which uses a dye as an indicator is the so-called "dye dilution " method. This method is briefly stated as follows: step a: arterial blood is withdrawn from the subject at constant speed via a catheter with a pump. The blood is made to flow through a recording spectrophotometric densitometer. This is an invasive time-consuming procedure performed by a trained physician assisted by a trained team of technicians, and it causes some pain and discomfort to the patient; and step b: a bolus of liquid containing a measured amount W of dye is rapidly injected into the blood circulation. The dye-arterial blood mixture flows through the densitometer and a recording of dye concentration as a function of time is obtained. The average concentration of the dye, C', in the arterial blood-dye mixture is then obtained from the relationship of dye concentration to time and the transit time T of the mixture permits the cardiac output to be obtained by the well known formula of Hamilton et al: EQU CO=W/(C'x T) (1)
To overcome the invasive and time consuming procedure of arterial blood withdrawal, step a), several investigators including M.McGregor et al in Circulation Research 9, pp. 1083-88(1961) and J. H. Reed, et al, J.Appl. Physiology 23, pp. 373-380(1967) used optical densitometers to measure and record the dye concentration in the arterial blood flowing in the pinna of the ear. The densitometers used for this purpose were modified ear oximeters used clinically to measure oxygen saturation.
This procedure was welcomed with enthusiasm in the field because of the noninvasive nature of the method of measuring concentration. However it was abandoned because of the many practical problems it presented including the need to vasodilate the earlobe and to calibrate the instrument.
A pulse oximetry apparatus for measuring the oxygen saturation of arterial blood is described in M. Konishi, T. Kisanuki, T. Yamanishi et al, U.S. Pat. No. 3,998,550 (1976). The oxygen saturation, O2Sat(0), is equal to the ratio, C(0)/{C(r)+C(0)}, that is the ratio of the concentration of oxyhemoglobin to the sum of the concentrations of the reduced form of hemoglobin and oxyhemoglobin, Hb=C(r)+C(0). The novelty of this method and apparatus rests on the recognition that, when the optical density of a living body is measured with instruments having appropriately fast time response, the pulsatile phase of the optical density is representative of the optical density of arterial blood only. As known from the prior art, the ratio of the optical densities of arterial blood at two different wavelengths permits one to calculate the oxygen saturation of blood. By measuring this ratio of the optical densities of arterial blood, Konishi, et al, as cited above laid the foundation for the method of measuring oxygen saturation which is called "pulse oximetry". This noninvasive procedure is used widely today clinically in health care facilities for monitoring oxygen saturation.
The invention described hereinbelow is based on application of the methods of Konishi, et al, to the problem of measuring the concentration of a dye injected into arterial blood in living tissue as a function of time.